One reason could be that someone has started doing sexual things with her against her will, and parents should be made aware of that. Or you will argue, I'm sure, that perhaps it was with her own enthusiastic consent. In today's society it is good for parents to know about that too.

If you have good evidence that the parents are horrible people, of course that complicates things.

And regarding this idea of betraying a child's confidence -- there are limits. Even if a 10-year-old says, "I'll tell you something if you promise you won't tell" and you say "OK", it can't be unconditional. If the secret is that she saw her neighbor burying a body in the back yard, for instance? Or that she is sniffing glue or shooting heroin? The more honest answer to that question is, "OK, unless it's dangerous".